At the DECHEMA
colloquium, held in Frankfurt on 11th Oct, 2001, Hug
and Barrett gave papers that expressed doubts about the details
of the manner that spectroscopy has been applied by members of
the IPCC in coming to their conclusions about future climate
change. IPCC proponents Bakan, Birk and Hollmann opposed the
views. This paper summarizes the main points of difference and
attempts to put forward the Hug/ Barrett arguments in the
clearest terms.

Itemized
reports

The mechanism of
global warming

The IPCC mechanism for
global warming relies entirely upon the theory of radiative transfer.
This assumes a long-term radiative equilibrium for the interaction
between incoming solar radiation and out-going long wavelength
radiation. The theory is correct for ‘blue skies’ and has to be
severely modified to take into account the effects of clouds and the
transfer of energy by non-radiative processes to the troposphere of
latent heat of evaporation of water. In particular, the IPCC
mechanism is based on the re-emission of absorbed terrestrial radiation,
some of which is directed downwards and causes the extra warming of the
Earth’s surface.

Hug and Barrett agree
that extra carbon dioxide in the atmosphere will further impede the flow
of radiation from Earth to space. They argue that the extra carbon
dioxide will also further impede the radiation returning to Earth from
where the atmosphere has absorbed it and that this factor is not
included in the IPCC mechanism. Hug and Barrett argue that the
energy budget indicates that the warming of the lower atmosphere is by (i)
heat transfer, (ii) latent heat transfer and (iii) radiation from the
Earth’s surface. The fractions of the heat flux producing atmospheric
warming1 by the three
processes are 19%, 61% and 20% respectively. The warming by radiative
means of only 20% arises as the resultant emission of 350 W m-2
from the surface minus the 324 W m-2 returning to
the surface from the atmosphere. [Of the 390 W m-2
emitted by a cavity radiator at 288 K, 40 W m-2 are not
absorbed by the atmosphere and reach space through the infrared ‘window’.]

The 20% warming by
radiation is more consistent with the Hug and Barrett contention that
the flux of 350 W m-2 absorbed by the lower atmosphere (by the molecules of water and carbon dioxide) is
converted mainly into the translational energy of the constituent
molecules, which thereby have an increased temperature. The
increased temperature of the lowest part of the atmosphere causes the
transfer of thermal energy to the higher parts by convection and the
evaporation of water. Certainly the air at higher temperature will emit
more radiation, but this seems to be of lower priority (20%) than the other two processes. The warming
mechanism operates every morning around the Earth and it is clear to Hug
and Barrett that at no time is there a true equilibrium in existence.

Barrett2 has presented similar arguments previously in
response to the obviously oversimplified explanation of the greenhouse
effect presented by the IPCC3.
His paper was criticised by Houghton4
and Shine5 who pointed
out some errors. These were replied to by Barrett.6 Braterman7 published a further criticism which
indicated that he did not understand the problem and his paper was
refuted by Barrett8 and
Courtney.9 The outcome of
this argumentation is that additional CO2 is expected to
increase global temperature, but the extent of the warming and the
mechanism of the warming are still debatable, as outlined above.

Applicability of
Kirchhoff’s Law

The above proposed
mechanism of terrestrial radiation being absorbed by the ‘greenhouse’
gases, producing rotationally and vibrationally excited states that are
then mainly degraded to their ground states by the conversion of their
excitation energy into the translational energy of colliding molecules
of dinitrogen and dioxygen has been criticised as violating Kirchhoff’s
law. This law specifies that a good absorber is a good emitter and
the IPCC supporters have interpreted this to indicate that they should
in equal measure emit the terrestrial radiation absorbed by the
greenhouse gases. This is true only for a system in thermal
equilibrium. The IPCC explains the phenomenon in terms of
radiation and imply that molecules do not obtain kinetic energy (heat) by absorption of radiation by greenhouse
gases. The IPCC state that the air is warmed by contact with the warmed
soil/ocean surfaces. The up-streaming of the atmosphere is caused by
such warming as are winds, storms, hurricanes and tornadoes. The IPCC
assumes that photons are recycled by CO2 causing 90% of
the absorbed radiation to return radiatively to the surface.1 Because the atmosphere is nearly
completely opaque in the CO2 region the re-emitted radiation can only ‘escape’
through the open IR window to space. These ideas were confirmed to Hug
at the DECHEMA colloquium. The misunderstanding arises from the IPCC
regarding the processes operating under conditions of true equilibrium,
where the Kirchhoff law operates, whereas under the non-equilibrium
conditions occurring daily in any part of the globe at any time, the law
is inapplicable. Under those daily circumstances, when warming of
the atmosphere is occurring, the molecules of the atmosphere absorb more
radiation than is emitted. As cooling takes place the reverse is the
case, radiative gases emit more than they absorb, acquiring their
excitation energy mainly by collisional processes.

Line broadening

Hug showed measured
spectra of (i) carbon dioxide, (ii) carbon dioxide and helium and (iii)
carbon dioxide with dinitrogen, where the sample cells contained the
same pressure of carbon dioxide and where the pressures of helium and
dinitrogen were both one atmosphere. The spectra are shown in Fig. 1

The three spectra differed
because of the different pressure broadening of the vibration-rotation
bands of the carbon dioxide in the three cases.10, 11 The integrated line strengths of
the band spectra are given in Table 1.

Table 1 Integrated line
strengths of CO2 spectra

In case (i) there was
self-broadening (resonance broadening) and in
cases (ii) and (iii) the broadening depended upon the nature (mass) of the ‘foreign’ gas. The extent of
broadening in all three cases was consistent with theory and showed that the
absorbing molecules of carbon dioxide did experience collisional interactions
that altered their spectral behaviour. The observations were consistent with
the Hug and Barrett contention that radiation absorbed by carbon dioxide was
most likely to be converted to the translational energy of colliding
molecules. Dr Birk particularly, supported by Drs Bakan and Hollmann, stated
that Hug’s observations were incorrect because of the relatively poor
resolution of the spectrometer used and that, in any case, the effects of ‘foreign’
gases were not as observed. The effects were due to instrumental errors and
that pressure broadening was a function of pressure only and not dependent
upon the nature of the ‘foreign’ gas. They claimed that the spectra
presented by Hug could be explained by deviations from the Beer-Lambert law
and showed a figure of a similar case. Fig.2 shows spectra of methane in the
presence of foreign gases.12

Fig. 2 Optical
density of methane at different pressures in the presence of foreign
gases

Hug and Barrett strongly
disagree about this view of pressure broadening and maintain that Hug’s
spectra show real effects. The resolving power of the spectrometer used
by Hug was exactly the same for all three spectra, so although somewhat
different observations would be obtained by using an instrument with a
different resolving power, the effects of the ‘foreign’ gases would
still be observed. It is not a Beer-Lambert law misinterpretation by Hug
since his measurements were carried out at constant pressure (1 bar) and at constant temperature (22 ° C). The spectra shown by the IPCC people
are measured at different pressures and are therefore not a proof of the
allegation that Hug had used the Beer-Lambert law incorrectly.
Note that in Fig.2 the optical density is given at a ‘peak’
wavelength of 7.65 m m and not as an integrated line strength over a
wider range (as in Table 1, for instance).
It has to be emphasized that in all three spectra the absorption of 1017
CO2 molecules cm-2(entrance to the sample cell) were measured. Thus
Hug’s spectra clearly show qualitatively that most of the infra-red
radiation absorbed by greenhouse gases is converted to kinetic energy.
This concerns mostly the main components of the atmosphere (N2 and O2). Therefore, the
exclusive photon recycling process on which all climate modelling is
based has to be rejected or modified.

The IPCC people used
another argument, saying that Hug’s spectra are affected by the effect
of slit-width (effectively the ‘Fourier
transform stop’) of the spectrometer used on the shape of the
spectral bands. This argument is also flawed because Hug used the same
spectrometer with the same parameters and thus any errors would be
consistent and relatively the same. The differences in the integrated
line strengths can only be explained in terms of the conversion of a
fraction of the absorbed radiation to the kinetic energy of the
molecules of the air.10

Atmospheric
sensitivity

Barrett gave figures for the
sensitivity of the atmosphere in terms of the warming in degrees K that would
be caused by an increase in atmospheric forcing of 1 W m-2. (i) The
accepted extent of global warming is 33°K, this arises form a forcing of 235 W
m-2 giving a value for the sensitivity of 0.14°K (W m-2)-1.
This is essentially a crude value because the relationship between forcing and
warming is not linear, but logarithmic. It would be expected that the
sensitivity to any enhancement of forcing would be considerably less than this
crude value. (ii) After the Mount Pinatubo eruption the Earth’s temperature
decreased by 0.3°K and the estimated reduction in forcing was 2 W m-2. This
gives a value of 0.15 (W m-2)-1 for the sensitivity. (iii) The Stefan-Boltzmann
equation linking the energy of emission of a cavity radiator to its
temperature:

E = s T4

may be differentiated with
respect to temperature:

dE/dT = 4s T3

and inversion gives a value for
the sensitivity:

dT/dE = 1/4s T3

If a value of 288°K (a mean value for the troposphere at sea-level)
is inserted into the equation the value for the sensitivity is 0.18 (W m-2)-1.
The three values produced are remarkably similar, but vary greatly from
the IPCC value obtained for the ‘business-as-usual’ scenario by
theoretical calculations of the effects of doubling the carbon dioxide
content of the atmosphere of a 2.5°K increase in temperature resulting
from an increase in forcing of 3.8 W m-2which gives the IPCC value for the sensitivity as
0.66 (W m-2)-1.

Spectral
contributions from CO2 and H2O

In their calculations of the
effects on the atmosphere of a doubling of carbon dioxide, the IPCC carry out
their line-by-line calculations and arrive at the value quoted above. They
ignore completely the spectral contributions from the water molecules present
in the atmosphere. Hug pointed out that the ‘wings’ of the carbon
dioxide bands were too weak to explain the predicted effects by the IPCC, but
the IPCC supporters claimed that there were no errors in the IPCC assumptions
and calculations. Hug emphasized that at a height above the ground of 1000 m
97% of the carbon dioxide bands are saturated in the current atmosphere, i.e.
transmission, T = 0. The IPCC claims that the absorption in these
bands are responsible for a greenhouse effect of 7.2°C and claims that a
further warming of 5.8°C will occur if the carbon dioxide level doubles. Hug
expressed his disbelief in these results, but the IPCC supporters confirmed
that the predicted warming would come to pass. Barrett showed the spectral
effects on a 100 m thick layer of atmosphere containing 360 and 720 ppmv CO2 in the
presence and absence of a pressure of water vapour equivalent to a relative
humidity of 50%. The results are given in Table 2.

Table
2 Results of HITRAN calculations

It is clear from the
calculations that there is a great amount of overlap between the spectra of CO2 and H2O and
that in the presence of water, the effect of doubling carbon dioxide is
considerably reduced compared to the effect in the absence of water vapour.
Hug and Barrett consider that the spectral overlap, ignored by the IPCC, is
the reason for the sensitivity being exaggerated by the IPCC and that the real
value is considerably less that that accepted by the IPCC. If Hug and
Barrett are correct, the effects of doubling carbon dioxide in the atmosphere
seem to be minimal and are no cause for alarm and the extensive alteration of
national economies.

The results presented by
Barrett were relevant to absorption of terrestrial radiation as dealt with in
the original paper3 that caused
so much argument. They do not represent the radiation fluxes at the top of the
100 m path, these would contain contributions from the upward thermal
radiation from that region of the atmosphere.

Water
(cloud) feedback estimations

All models of the atmosphere
are in agreement that a doubling of carbon dioxide leads to some increase of
the mean temperature, although the magnitude of the increase depends upon the
particular model used. The increase in water vapour present in the warmed
atmosphere leads to greater cloud cover and this alters the mean temperature,
this is described as the water vapour feedback. The various models in use give
very varied values for the water vapour feedback and do not agree on the sign
of the value. If the MRI (Japan) model values are
ignored, because they are clearly in error, the values for the feedback vary
from the BMRC (Australia) figure of –1 W m-2
to the LMD (France) figure of +1.75 W m-2 as shown13 in Fig. 3.

Fig. 3 The results from some models for
the effects of extra cloud forcing on solar,
terrestrial and net radiation for the doubling of atmospheric CO2.

The extent of this variation
does not give much confidence in the current state of modelling, doubts
which must extend to the modelling of the overall sensitivity and to the
general prediction capability of models in general.

Local
thermodynamic equilibrium

Local thermodynamic
equilibrium is a concept that is misunderstood by some. Its normal
interpretation is that the collision rate in the system is sufficiently
high to ensure that the equipartition principle holds, i.e. that all
available modes are populated according to the appropriate value of the
Boltzmann factor. This does not mean that any part of the system is in
thermodynamic equilibrium with its immediate surroundings. One cubic
metre of the atmosphere may very well be in LTE and the adjoining six to
fourteen cubic metres could also be in LTE, but it is not necessary for
all the systems to be in true thermodynamic equilibrium with each other.
If this were to be the case throughout the atmosphere, there would be
true thermodynamic equilibrium and nothing would change. The atmosphere
is not in true thermodynamic equilibrium and is constantly changing,
although in the regions below 90 km altitude local thermodynamic
equilibrium holds. Only above such altitudes does LTE break down as the
natural lifetime of excited states of the greenhouse molecules are then
longer than the rates of collisional excitation and the emission rates
are governed by the rates of collision.

The
impossibility of the doubling of atmospheric CO2

Barrett referred briefly to
the sink mechanisms of atmospheric CO2 and pointed
out that at the current rate of burning fossil fuels that it would be
expected that the CO2 concentration would level out in about 40-50
years at between 420 and 460 ppmv. He assumed that the composite rate
constant for the sink processes was that which was consistent with a
half-lifetime of 38 years, far less than the figure quoted by the IPCC14 of ‘several hundred years.’
The kinetics are those concerned with the establishment of a
quasi-equilibrium between atmosphere and the immediate sinks, i.e. the
biota and upper layer of the oceans. The re-establishment of the ‘true’
equilibrium pre-industrial era concentration of 285 ppmv of CO2
would be expected to take a much longer time than that quoted by the
IPCC, and would only occur if fossil fuels had been replaced as the
major energy source. The chairman indicated that discussion of the
carbon cycle was not the purpose of the meeting, but a member of the
audience disagreed with Barrett’s argument and stated that he
expected the CO2 content of the atmosphere to ‘go up and up.’
Clearly, another meeting is required to explore this topic in some
detail. If CO2 in the atmosphere is not going to reach the
level represented by the doubling of the pre-industrial value, the
whole basis of the IPCC calculation of the future climate is
incorrect.

Conclusions

The mechanism of global
warming must be defined in a better manner so that all relevant
processes are represented.

It must be recognized that
Kirchhoff’s law applies only to systems in thermal equilibrium.

The proper effects of line
broadening must be recognized. That line broadening occurs at all
indicates that collisional processes are important.

The value of the
atmospheric sensitivity given by the IPCC seems to be too large. If
the IPCC value for atmospheric sensitivity is exaggerated by a factor
of about four, the predicted increase in temperature for a doubling of
carbon dioxide will be too small to be discernible against the
background of natural variability of the climate.

The ignoring of the
spectral effects of water vapour seems to be the reason for the
exaggeration of the sensitivity.

The failure of models to
settle the sign of the water vapour feedback induces doubts about the
whole process of modelling at the present time.

The concept of local
thermodynamic equilibrium is given different interpretations and leads
to errors in applications of theory to the atmospheric problem.

The kinetics of the carbon
cycle needs further consideration. If the CO2 content of
the atmosphere is not going to reach double that of the pre-industrial
era, the whole of the IPCC approach to future climate change is in
doubt.

The authors would be
grateful for any feedback and discussion on any of the points raised in
the paper.

Editor Note:This paper is
now the subject of an `open review'
and comments are invited by email. The Open Review is published
here. Emails
for `open review' should be sent to openreview@john-daly.com
, with the title `Hug-Barrett Paper' in the subject line.

Comments should
be relevant to the subject matter of the paper, and normal
courtesy all round would be much appreciated.